Place the biscuits in a polythene bag and bash with a rolling pin to form fine crumbs. Divide between 4 tall serving glasses and use to line each base.
Mash one of the bananas and divide between the 4 glasses, spooning on top of the biscuit crumbs.
Melt the butter in a small saucepan, add the sugar and heat over a medium heat, stirring well, until the sugar has dissolved. Add the cream and cook gently for 1–2 minutes until the mixture is thick. Remove from the heat and leave to cool for 1 minute, then spoon on top of the mashed banana.
Slice the second banana and arrange on top of the caramel, then spoon over the crème fraîche. Decorate with grated plain dark chocolate before serving.